Conventional techniques for producing photovoltaically active semiconductors (PVAS) for use in solar cells are normally costly, complex or dangerous. In addition, many conventional solar cells are fabricated with a relatively thick PVAS, much of which is not actually active in the photovoltaic process. As a result, the cost of producing solar cells has not been reduced to a level which can make them a competitive alternative to nuclear or fossil fuel for producing electricity.
The most popular commercial solar cell is the single crystalline silicon cell. This type of cell utilizes a silicon material that is relatively expensive to produce. However, because of the brittle nature of the silicon it is difficult to produce slices of this material less than twenty thousandths of an inch thick. Therefore, in use in the solar cell 99.9% of this expensive material acts as a substrate, and only the top 0.1% of the wafer is actually utilized as a PVAS. Even this 0.1% layer is relatively thick because the single crystal silicon is an indirect bandgap semiconductor, which requires a thickness of up to one hundred times more than a comparable direct bandgap semiconductor to absorb an equal number of incident photons.
In order to reduce the cost of solar cells, the use of direct bandgap PVAS materials has been widely studied. Many of these direct bandgap materials can be utilized to fabricate a relatively efficient solar cell with a total thickness of the PVAS of only on to two microns. This extremely thin material can be deposited on a suitable substrate, which provides rigidity and flatness, to produce a low-cost thin-film solar cell.
There are several techniques for producing thin-film solar cells, many of which exhibit cost or scale-up problems. Flash evaporation is not suitable for large area deposition and is very material inefficient. Chemical spray pyrolysis is not applicable to all materials and results in low quality films Multi-source evaporation is expensive and not suitable for large area coverage. Molecular beam epitaxy is an extremely expensive deposition technique. In contrast, sputtering is a very inexpensive technique for large area deposition of a thin-film PVAS.
In addition to the problems associated with production techniques, there are also problems associated with many of the materials which are suitable for use as thin-film solar cells. Amorphous silicon is easy and inexpensive to deposit but has reduced efficiency and problems with long-term stability. Copper sulfide is also a relatively unstable compound. Gallium arsenide is difficult and expensive to produce in the required single crystalline form. In contrast, copper indium selenide is an excellent solar cell material because cells made from it are very stable, it is a direct bandgap material, has a higher absorption coefficient than most direct bandgap materials and operates effectively in polycrystalline form.
As a result of the above constraints, sputtering of copper indium selenide has been studied as a promising method of manufacturing thin-film solar cells. Conventional sputtered copper indium selenide solar cells are made by sputtering a p type layer of copper indium selenide and then depositing an n type layer of another compound, typically cadmium sulfide, on the surface of the copper indium selenide. This technique produces an efficient solar cell heterojunction, but exhibits two substantial problems. First, the copper indium selenide is generally produced using a reactive selenide gas and sputtering only the copper and indium because of the difficulties of sputtering a ternary compound. This reactive sputtering technique typically uses hydrogen selenide as the reactive gas. Hydrogen selenide is an extremely toxic, odorless and colorless gas which can only be used with expensive containment chambers and safety apparatus. Thus, this reactive sputtering system is inappropriate for high-volume production of solar cells.
The second problem associated with traditional methods of producing these thin-film heterojunctions is that two separate deposition systems are required to deposit the two different semiconductor compounds. The p type copper indium selenide is produced first in a sputtering chamber. The n type cadmium sulfide layer is usually then deposited in an independent evaporative vacuum system. It has been shown that losses in efficiency occur due to exposure of the surface of the copper indium selenide, which forms the heterojunction, to air during transfer to the cadmium sulfide production chamber. Thus, the two systems required to produce this p/n heterojunction are expensive and may result in a thin-film solar cell with reduced efficiency.
A single sputter deposition system using only one PVAS compound to produce a thin-film homojunction would thus seem to be the ideal method of fabricating a solar cell. Conventional techniques for sputter deposition of thin-film homojunctions, such as U.S. Pat. No. 4,057,476, vary the carrier type of the deposited film layers by varying only substrate temperature, film growth rate, or a bias voltage applied to the substrate. Because of the problems associated with sputter deposition of copper indium selenide, these conventional sputtering techniques use other compounds, such as Lead Tin Telluride, which may not be PVASs and thus may not possess the advantages of copper indium selenide. Thus, it is highly desirable to develop a simple and inexpensive technique for sputter deposition of copper indium selenide to form a PVAS homojunction for use in solar cell technology.
Once developed, this sputter deposition method can be used to produce a variety of semiconductor homojunction products by taking advantage of the relative ease and low cost of using sputtering to deposit semiconductor materials, as opposed to the other deposition techniques discussed above. Thus, semiconductor products with two or more layers, and made with one or more semiconductor materials, may be readily fabricated. These structures may include, for instance: transistor devices, high-efficiency multiple-layer sequential homojunction solar cells, radiation detection devices, light-emitting diodes, and, of course, thin-film solar cells.